Gas turbine engines generally include a gas generator which comprises a compressor for compressing air flowing aft through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a turbine driven by the gas stream and connected for driving a rotor which in turn drives the compressor. Many engines further include a second turbine, known as a power turbine, located aft of the gas generator and which extracts energy from the gas stream to drive propulsor blades. Such engines may be, for example, ducted turbofan engines and turboprop engines. Vibration isolators have been used with these engines to attenuate vibrations from the rotating propulsors and transitory shock due to maneuvering of an aircraft powered by the engines.
A recent improvement over the turbofan and turbo-prop engines described above is the unducted fan engine such as disclosed in U.S. patent application Ser. No. 437,923 -Johnson, filed Nov. 1, 1982. In the unducted fan engine, the power turbine includes counter rotating rotors and turbine blades which drive counter rotating unducted propulsor or fan blades radially located with respect to the power turbine. As with any turbofan engines, there is a remote possibility that an unducted fan blade may be damaged or even lost if struck by an object such as a large bird or debris. A lost or damaged fan blade will result in an extreme imbalance condition and cause severe vibration of the engine.
The vibration potential of blade damage in these engines have required the development of mounting arrangements which can withstand vibration and prevent an engine from being torn loose from an aircraft by the vibrations. An example of a mounting arrangement for a turbofan engine is shown in U.S. Pat. No. 3,056,569 to Bligard which illustrates a two-plane mounting arrangement for attenuating ordinary vibration to the engine as well as some transitory vibration caused by maneuvering of the plane. The mounting arrangement employs two side mounts positioned on opposite sides of the engine and two top mounts. One of the top mounts is positioned in a common axial plane with the two side mounts. The mounts are arranged to provide their greatest restraint against forces which act in a direction along the main axis of each mount.
It is believed that one disadvantage of such prior art engine mounts is that they are dynamically indeterminate, i.e., under any given stress condition, it cannot be determined which mount will support the major portion of the stress. Additionally, any thermal expansion of the engine will vary and create stresses in the mounts. It is believed that another disadvantage of the prior art mounts is their inability to dampen vibrations in more than one direction. Thus, forces acting on a mount in a direction other than its main axis may result in a transfer of vibration to the aircraft itself.